1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly to techniques for correcting position difference among a plurality of images.
2. Related Background Art
A color image forming apparatus is known which has a plurality of record units each applying a laser beam modulated by record data to a photosensitive drum, developing an electrostatic latent image on a photosensitive member by an electronic photographing process and transferring the image to a transfer sheet to form a color image by sequentially transporting the transfer sheet by a transfer belt to each record unit and superposing each color image on the transfer sheet.
With the color image forming apparatus of this type, each electrostatic latent image on each photosensitive drum is developed and transferred to a transfer sheet on a transfer belt. In this case, registration of each color image is shifted if there is a mechanical mount error of each photosensitive drum, or an optical path length error or an optical path change of each laser beam.
In order to solve this problem, so-called registration correction techniques have been proposed. According to the techniques, a registration correction pattern image formed on a transfer belt is read with a CCD sensor or the like to detect a registration shift of each color photosensitive drum. In accordance with this registration shift, electrical correction of image signals to be recorded is performed and mechanical correction of the optical path length or optical path is performed by driving each reflection mirror disposed in the intermediate optical path of each laser beam.
The image forming apparatus of this type is, however, associated with some problem in registration correction that the read level of each registration correction pattern may become different and hence the detection level of each registration correction pattern may become different. This problem results from a different image density of each registration correction pattern recorded by the record unit, i.e., a different composition of coloring transfer material and the like.
If electrical correction of image signals to be recorded is performed and mechanical correction of the optical path length or optical path is performed by driving each reflection mirror disposed in the intermediate optical path of each laser beam, in accordance with the registration shift amount obtained by detecting each registration correction mark recorded by the record unit, then high precision adjustment is difficult because the detected registration shift amount itself may contain some error.